1. Field of the Invention:
This invention relates to an apparatus for recording and reproducing information on and from an optical disk using an optical beam, and an optical disk for storing information useful in such an apparatus. In the specification and accompanying claims, the term "an optical disk" includes an optical disk in which information cannot be rewritten (e.g., a so-called compact disk) and also that in which information can be rewritten (e.g., a magnetooptical disk), and the term "an apparatus for recording and reproducing information" means an apparatus which can record and/or reproducing information on and from an optical disk.
2. Description of the Related Art:
In an apparatus for recording and reproducing information on and from an optical disk using an optical beam, tracking servo control is performed so that an optical beam such as a laser beam accurately traces recording tracks preformed in the optical disk. In order to perform the tracking servo control, the continuous tracking servo method is widely employed which uses servo control signals obtained from guiding grooves or data pits preformatted in an optical disk for guiding a laser beam along the recording tracks.
An optical disk used in the continuous servo method has preformed continuous grooves or data pit rows so that the servo control signal can be easily obtained from the grooves or pit rows independently of the data recording density or recording method. Therefore, this method can be applied to optical disks with various formats.
When using an optical disk on which data is recorded based on variations in the reflectivity of the recording medium or the presence and absence of pits, however, the continuous servo method does suffer from deterioration of the servo control signal obtained from the grooves or pit rows near the recording areas, due to the effect of the recorded signal. Furthermore, the quality of the servo control signal tends to be affected by slight dimensional inaccuracies such as shape errors in the guide grooves. Therefore, it is difficult to obtain compatibility between various disk types.
As another method for performing the tracking servo control, the sample servo method is employed which uses a tracking servo control signal obtained from servo bytes preformatted in sampling areas which are formed locally on the optical disk. In the sample servo method, a phase lock loop (PLL) circuit generates a reference clock signal in synchronization with the timing with which an optical beam passes the servo bytes. Based on this reference clock signal, a tracking servo control signal is obtained from the servo bytes, and the timing with which the data signal is written and/or read is controlled.
In the sample servo method, the sampling areas are completely separated from the areas in which data is recorded, and therefore the tracking servo control signal is not affected by the recording signal and a tracking servo control signal of high quality can be easily obtained. Furthermore, the formation of sampling areas specialized for the tracking servo control allows the apparatus to use a relatively simple detection means, resulting in that the sample servo method can offer a wider range of disk compatibility than the continuous servo method.
When a focus servo control is performed against an optical disk having guide grooves in the astigmatic method, the presence of the guide grooves may cause a shadow to form in a beam spot on an optical detector, thereby necessitating the precise positioning of the optical detector. Since it is not necessary to form guide grooves in an optical disk used in the sample servo method, in contrast, such a shadow does not form even when a focus servo control is conducted. This results in that the servo control can be accurately executed without excessively accurate positioning of an optical detector. Therefore, the sample servo method offers advantages such as reduced steps in the assembly of the detector and an improved yield of an apparatus.
Generally, an optical disk is driven under either the constant angular velocity (CAV) control in which the rotational velocity of the disk is controlled so that its angular velocity is kept constant, or the constant linear velocity (CLV) control in which the rotational velocity of the disk is controlled so that the speed of the optical beam spot with respect to the optical disk (i.e., the linear velocity) is maintained constant.
While the CAV control is simple since the rotational velocity is kept constant irrespective of the irradiation position of the optical beam on the optical disk, the CAV control involves a disadvantage in that the recording density at the portion nearer the outer circumference of the disk becomes smaller, thus reducing the storage capacity of the disk as a whole.
In the CLV control, on the other hand, the storage capacity can be easily increased, but the rotational velocity of the optical disk must be varied in accordance with the change of the radial position of the optical beam spot on the optical disk. Moreover the extra time required for stabilizing the rotational velocity makes the access time longer.
To tackle these problems, the modulate-constant angular velocity (M-CAV) control has been proposed in which the rotational velocity of the optical disk is kept constant, the recording area of the disk is divided into a plurality of blocks comprising multiple tracks, and information is written and/or read using a clock signal having a higher frequency in the more outer block. According to this M-CAV control, it is possible to avoid the increase in the access time caused by the change of the rotational velocity, and in addition, the linear recording density near the outer circumference of the optical disk is not reduced, so the storage capacity can be easily increased.
However, in the M-CAV control, it is difficult to employ the sample servo method which has various advantages as described above, because of the reasons mentioned below.
That is, in the M-CAV control, the period with which the optical beam passes a servo byte changes with each block. Therefore, when a sample servo method is used, each time the optical beam moves in the radial direction of the optical disk and enters into another block, the tracking control cannot be correctly performed until the PLL circuit is pulled again into synchronism.
In the sample servo method, comparison pulses input to the PLL circuit per unit time is small in number so that, if the period with which the optical beam passes the servo byte changes greatly, the response time is prolonged until the PLL circuit is pulled again into synchronization to generate a stable reference clock signal. Consequently, even though the M-CAV control method does not require the change of the rotational velocity of the optical disk, it does result in longer access times.
When performing the M-CAV control in the sample servo method, therefore, it is difficult to shorten the overall access time while increasing the storage capacity of an optical disk.